1. Field of the Invention
The present invention relates to a lens system for focussing a divergent laser beam and more particularly for converting the three dimensional amplitude and phase distribution of the highly divergent laser radiation of a semiconductor laser into the amplitude and phase distribution of the fundamental mode coupleable into and conducted in a mono mode fiber.
2. Description of the Prior Art
When constructing semiconductor laser modules, a key problem lies in the optical coupling between the laser diode and the monomode fiber to be coupled-on. The coupling should optimally transform the three-dimensional amplitude and phase distribution of the laser diode emission into the amplitude and phase distribution of the mode conducted in the fiber in order to obtain a good coupling efficiency.
Macro-optical and micro-optical arrangements have been disclosed as coupling arrangements. Micro-optical arrangements are, for example, the butt coupling between laser and end face of the fiber which, in the simplest case, is planar or arched in the form of a micro-lens, whereby the fiber end can additionally have a taper shape. Gradient lenses can also be employed instead of fused or fused-on micro-lenses.
In the known macro-optical coupling arrangements, single-lens or multi-lens systems, particularly lens systems of the type initially cited are used, their typical dimensions being large in comparison to the fiber or a corresponding waveguide (see O. Krumpholz, Wiss. Ber. AEG-Telefunken 48 (1975) pp. 90-94 and S. Masuda, T. Iwama, Appl. Opt. 21 (1982) pp. 3475-3483). The simplest case in this category is the coupling with a spherical lens (see Sugie et al., J. Lightwave Techn. 1 (1983) pp. 121-130). Condensor-like lens systems of the type initially cited are, however, often employed.
In such coupling arrangements or systems, the coupling efficiency of 50% could usually not be exceeded because none of those arrangements could handle the great divergency of the laser radiation emitted by standard semi-conductor lasers. (see Saruwatari, Sugi, Journ. of Quant. Electr. 17 (1981) pp. 1021-1027).
With the use of these known arrangements or systems, the reaction on the laser resonator typically lies in the per thousand range because of the slight distance of the first refractive surface from the laser mirror. Antireflection coating would in fact reduce the reaction but would further increase the losses at some other location due to the highly curved surfaces.